A sealed secondary battery in which a power generating element is hermetically sealed in a case is heretofore used in a vehicle, an electric device, and others. For instance, there is a lithium ion secondary battery in which a power generating element including positive and negative electrode plates and electrolyte is contained in a flat rectangular case. A method of producing such battery may be achieved by inserting the electrode plates in the case and then sealing an opening of the case by a closing plate. After that, the electrolyte is poured through a liquid inlet provided in the closing plate. In this case, the liquid inlet has to be hermetically closed after pouring of the electrolyte.
For hermetically closing the liquid inlet, one conventional method is achieved by placing a closing member on a liquid inlet and then welding the closing member to the liquid inlet by laser. For instance, as shown in FIG. 13, the peripheral edge of a closing member 102 covering a liquid inlet 101 is welded thereto sequentially by laser, thereby hermetically closing the liquid inlet 101. However, as disclosed in Patent Literature 1 for example, blowholes might be generated in a welded portion. The blowholes are caused by gas generated during laser welding. In this Patent Literature 1, a main cause of the gas generation is considered as climbing or creeping up of electrolyte. Therefore, a water repellent agent or an oil repellent agent is coated on the vicinity of a laser welded portion. This is assumed to prevent generation of blowholes.    [Patent Literature 1] JP2000-106155A